WO 03/060212 A1 discloses a hybrid cord for use in pneumatic tires, designed for use as an overlay. The hybrid cord is formed of two different materials: a low initial modulus core yarn and high modulus wrap yarns. The selection of the yarns is such that the “break point” of the cord, i.e. when the slope of the force versus elongation curve changes from a relatively low slope to a relatively high slope, occurs at an elongation between 2% and 3% elongation, with an ultimate cord break at just over 5% elongation.
U.S. Pat. No. 6,799,618 B1 discloses a hybrid cord for use as an overlay in pneumatic tires. The hybrid cord is formed of aramid and nylon twisted together, wherein the break point of the cord is at an elongation between 4% and 6% elongation, with an ultimate cord break at over 10% elongation.
The above publications describe the use of a hybrid cord especially for the overlay. In an overlay, the hoop reinforcing effects of a strong cord are desired, however, the cord must have elongation properties to a degree to permit the tire to expand into a toroidal shape during tire molding. These were, heretofore, contrary properties when using a single material cord.
In U.S. Pat. No. 6,695,025 B1 (Roesgen) discloses a run-flat tire having two carcass reinforcing plies and reinforcing wedge inserts in the tire sidewalls. As noted in Roesgen, the wedge inserts resist radial deflection of the tire with a combination of compressive and bending stresses in both the inflated as well as the uninflated conditions. Runflat tires experience a net compressive load in the region of the sidewall closest to the road-contacting portion of the tire. Additionally, the outer portions of the sidewall experience tensile forces while the inner portions of the sidewall undergo compression stresses during bending. Roesgen solves the issue of balancing the necessary flexibility in the inflated state with the rigidity in the uninflated state problem by employing two reinforcing carcass plies. The axially outermost ply has cords that have a modulus of elasticity that increases with strain. The axially innermost ply has cords having a modulus that exceeds that of the outermost ply during normal loads in an inflated state; thus the innermost ply handles the majority of the load during normal operation, and the outermost ply does not equally contribute to the load carrying. When the tire is operated in uninflated states, the load is shifted from the axially innermost ply to the axially outermost ply and again the plies do not equally contribute to the load carrying. Roesgen teaches that the outermost ply does not contribute to the overall rigidity of the tire sidewall during normal inflation operation.